the present invention relates to a multi-purpose valve for use as a therapeutic device, comprising a band having when in situ the shape of a conical spiral formed by multiple, successive, helical turns, in such a way that at a first pressure difference across the valve, the helical turns of the band abut one another and seal the cone, whereas at a second pressure difference across the valve, which is larger in the direction towards the tip of the conical spiral than the first pressure difference, the valve is extended axially such that said turns separate from another so as to allow flow therebetween.
The multi-purpose valve according to the invention is designed for the treatment of chronic, venous insufficiency secondary to primary valvular incompetence or post-thrombotic syndrome. Furthermore, the device can act as an inexpensive alternative to artificial heart valves which are in use and as a sphincter at cxaravascular locations such as the urethra and the pylorus.
Venous valves are crucial for normal blood flow in the lower extremities. Damage to these structures is a common result of thrombosis, leading to the development of post-thrombotic syndrome, a common, chronic and often disabling disease.
Surgical reconstruction of venous valves is a technically demanding procedure and therefore has achieved only limited acceptance in clinical practice. Thus there is a clear need for an artificial valve which can be implanted in the veins.
In addition to this, there is a need for artificial heart valves which can be implanted through the skin, and sphincteric devices which can be placed in the urethra and the pylorus.
Some bioprosthetic valves have been developed for use as venous valves (e.g. U.S. Pat. No. 5,500,014) but none of them has been found suitable for clinical application. These devices consist substantially of a rigid metal ring covered by a biocompatible polymer. Heart valve flaps from pigs or flaps designed from bovine pericardium which are attached by means of glutaraldehyde are secured by sutures to the metal ring. The main advantage of these devices is that they are based on a concept which has been tried and tested in the heart. However, this advantage must be weighed against the risk which is involved in using animal tissue. Bioprosthetic valves, moreover, have a limited life due to the degenerative changes which inevitably follow implantation. The bioprosthetic valves have only been tested as venous valves in vitro or in the largest central veins in non-primate mammals. Thus their real thrombogenic potential is still Sown.
Another known valve is a metallic flap disc valve (developed at Millard Fillimore Hospital by Taheri et. al.). This valve has been shown to be unsatisfactory.
DE-A-4204138 (Zimmermann et. al.) describes an artificial valve comprising two or more wires in spiral configuration which are attached to a rigid ring. While this invention is free of some of the drawbacks listed bove, its function is dependant on arterial haemodynamics, making it unsuitable as a substitute for venous valves. In addition, the ring precludes the possibility of implanting the valve by a non-surgical percutaneous approach.
These and other problems related to the known devices are solved by means of the multi-purpose valve according to the invention. This valve is characterized in that the valve is constituted by one single band of a thermodynamic metal alloy with shape memory, with said conical shape as a memorised shape, and with transition temperature in the range for the normal body temperature of a mammal, (35-41 degrees Celsius) with the result that the band has a linear shape below the transition temperature and assumes the memorised, conical shape above the transition temperature, making the valve percutaneously implantable.
In a further preferred embodiment of the invention, the band is provided on one surface with flexible, highly magnetic strips and on the opposite side with ferromagnetic strips. This feature permits prostheses to be manufactured with different opening pressures simply by altering the pole strength of the magnetic strips.
The valve according to the invention is designed by winding a thin, narrow band of a thermodynamic metal alloy with shape memory (e.g. Nitinol) round a mould with a suitable shape and heat treating it. The alloy is characterized by long-term stability under cyclic loading, and its transition temperature lies below the normal body temperature, All metallic surfaces are preferably covered with a biocompatible, bistable, non-thrombogenic polymer layer. Below the transition temperature range, the thermodynamic metal is in a martensitic state and the device has a linear shape. Above the martensitic transition temperature range the metal band is transformed to austenitic state, thereby recovering the memorised shape which it received during heat treatment.
Another important feature of the invention is that it permits transcatheter implantation through the skin, on account of the band""s linear shape before it reaches normal body temperature. The band with its linear shape can easily be inserted in the body and subsequently, when the transition temperature is reached, it will assume a valve shape.
The valve according to the invention can be manufactured from freely available materials, and it does not include animal tissue of any type. This eliminates the risk of transferring zoonotic infections and of causing undesirable immune reactions.
In a preferred embodiment the whole device is covered with a biocompatible, non-thrombogenic, biostable polymer (e.g. low-molecular-weight dimer of parachloroxylene (parylene), thus eliminating the risk of chemical interaction between the substrate and the blood. Human cells proliferate rapidly and easily on the parylene-coated surface to produce thin adhesion layers of morphologically normal tissue. Thus rapid endothelium cover of the prosthesis according to the invention can be predicted, thereby minimising the risk of de novo thrombosis.